109. Convert Sorted List to Binary Search Tree

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Problem

Given a singly linked list where elements are sorted in ascending order, convert it to a height balanced BST.

For this problem, a height-balanced binary tree is defined as a binary tree in which the depth of the two subtrees of every node never differ by more than 1.

Example:

Given the sorted linked list: [-10,-3,0,5,9],

One possible answer is: [0,-3,9,-10,null,5], which represents the following height balanced BST:

      0
     / \
   -3   9
   /   /
 -10  5

Code

class Solution {
    public TreeNode sortedListToBST(ListNode head) {
        if(head == null) return null;
        return helper(head, null);
    }

    private TreeNode helper(ListNode start, ListNode end){
        if(start == end) return null;

        ListNode slow = start;
        ListNode fast = start;

        while(fast != end && fast.next != end){
            slow = slow.next;
            fast = fast.next.next;
        }

        TreeNode root = new TreeNode(slow.val);
        root.left = helper(start, slow);
        root.right = helper(slow.next, end);
        return root;
    }
}